This invention relates to method for preparing a dental prosthesis. This invention also relates to an apparatus or system for use in carrying out the method. In addition, this invention relates to a mold assembly for preparing a dental prosthesis. More particularly, this invention relates to a method, an associated apparatus or system and a mold assembly for manufacturing substructures, particularly metal or alloy copings, of prosthetic dental overlays such as crowns, bridges and splints.
In accordance with conventional techniques for providing a dental patient with a prosthetic overlay such as a crown, bridge or splint, a dental practitioner grinds the subject tooth or teeth down to form one or more tooth preparations to which the prosthetic device is to be attached. An impression of the tooth preparation or tooth preparations is taken in an elastic material and the impression is used to produce a model with dies. This in turn has a wax shape built to fit the die, so that a metal casting can be processed via the lost wax technique. The metal casting is then provided with a porcelain layer.
This method of casting for manufacturing prosthetic dental devices is labor intensive and, accordingly, expensive. In addition, the time required to make a dental prosthesis by such labor intensive methods is substantial and thus results in considerable delay in providing patients with crowns, bridges and splints.
The metal castings for the prosthetic devices are generally made by dental laboratories from metals or alloys purchased in the form of small ingots. After applying porcelain cover layers to the metal castings, the laboratories ship the finished prothetic products to the dental practitioners who ordered them.
The manufacture of customized dental prostheses entails substantial efforts and time expenditures by dental laboratories to customize the fit of the castings, resulting in a reduction in value of the precious metal and a using of amounts of precious metals in the process that is lost in castings, grindings. The casting system is subject to so many variables as waxing thicknesses, investment expansions, metal homogenities that there are necessarily inaccuracies and errors in castings results, increased expense in the delivery of what is required and delay in the finalized, correctly fitted prosthetic dental devices. In addition, it is frequently necessary for the dental technicians to hand shape required margins as well as eliminate small bubbles from the metal castings and to grind both internal and external surfaces of the prosthetic appliance in preparation for use and insertion in in the patients, mouths. This grinding away of expensive precious metal or any other metals is time consuming and results in inaccuracies, modifications of fit, amd higher costs for precious metals.
In producing bridges or splints pursuant to traditional methods, the bridges or splints are frequently fabricated by using the excess materials of several prior castings, these prior castings being the excess of the sprued units. This manufacturing technique, as discussed above, is labor intensive and therefore results in high costs. In addition, in cases where there are soldered joints in a prosthetic dental device there is an unequal distribution of stress responsive forces throughout the device, and as a consequence multi-unit cases may be subject to failure due to porosity and/or fatigue at the soldered joints. Moreover, gases are generated in the casting and/or soldering process and such gases remain in the metal and are released and weaken the procelain when that material is baked onto the metal.
Because handheld grinding and/or drilling of the metal castings naturally results in reduced accuracies in the shapes of the final products and because conventional techniques for manufacturing dental prostheses such as crowns, bridges and splints are subject to continuous variables from the impression stage, to the modeling, waxing, casting and handheld grinding, a goodly number of dental prostheses are frequently ill-fitting or require multiple corrective steps, which gives rise to further variables, delays and cost increases.
New methodologies based on CAD/CAM and CAE design have recently been introduced. These new methodologies represent the only significant advance in the dental arts for centuries. Pursuant to the new techniques, a dental prosthesis such as a bridge is machined or milled from a solid chunk of material under computer control. The milling is of both internal and external surfaces of adjacent dental substructures and proceeds generally from tooth position to tooth position. Upon reaching the final tooth position in a bridge array, numerous inaccuracies have arisen from the extensive milling or grinding.